I m now persuing my 7th sem B.E and I m willing to do M.S but not yet prepareded for any of the exams like GRE,TOFFLE,when to start ?

You are not late and after finishing your 8th sem exams, you can appear for GRE and TOEFL. During the admission process, you will have to send all your academic transcripts(mark sheets) including final sem, two letters of recommendations, an essay called-'Statement of purpose' and registration fees.